1. Field of the Invention
The present invention is related to calibration circuits and, in particular to calibration of proximity and touch detectors.
2. Background Information
Touch detectors have been in use for some time to indicate a condition where there is contact between the detector and some other solid object. Any well-known electromechanical xe2x80x9con/offxe2x80x9d switch can be used in a touch detector. Proximity detectors also have been used for some time to indicate when one object is close to another object and to measure how far away one object is from the detector.
xe2x80x9cCapacitivexe2x80x9d sensors often are used in proximity detectors and in touch detectors because electromechanical limit switches tend to break or malfunction over time from use. Capacitive touch sensors translate effective capacitance to a binary signal, whose state determines whether that effective capacitance has been exceeded. The effective capacitance relates to a distance between an object and a sensor plate. This distance may be separated by a dielectric, such as a polycarbonate.
There are a variety of well-known ways of measuring capacitance between two objects. One way is to use an oscillator in which the unknown capacitor at the input of the oscillator is charged and discharged between two distinct thresholds. When the voltage input to the oscillator reaches one threshold value, the output of the oscillator switches to a low value. When the voltage input to the oscillator reaches the other threshold value, the output of the oscillator switches to a high value. The frequency of the oscillator is dependent on the magnitude of the unknown capacitance, i.e., dependent on how fast the unknown capacitor is charged and discharged.
Proximity detectors require frequent calibration to compensate for component aging, temperature variations, power supply deviations, and other environmental changes. Manual calibration or factory calibration should not be necessary as they are time consuming and costly. What is needed, therefore, is a touch sensor that is capable of calibrating itself automatically and periodically.
Presented herein is a circuit and method to calibrate a proximity/touch detector. One aspect of the present invention distinguishes between an object, such as a finger, almost touching a front panel of a computer and the object actually touching the front panel. The present invention modifies (switches in) input capacitance to establish a quiescent operating point for an oscillator, where the change in output frequency is proportional to the change in input capacitance that would result from a front panel being touched from a condition of being free from contact.
An advantage of this and other aspects of the present invention is that initial factory calibration and periodic manual calibration are not needed. Other features and advantages as well as the structure and operation of various embodiments are described in detail below.